The present invention relates to the reduction of distortion in electrical circuitry. More particularly, the present invention relates to distortion reduction circuitry particularly suitable for amplifiers.
Radio frequency (RF) systems, such as cellular and microwave communication systems, typically require input signals to be fed into an RF power amplifier. Unfortunately, all amplifiers tend to distort the input signal at some power level and to generally cause the amplifier output to contain undesirable distortion products, spurious products, noise perturbations and intermodulation (IM) products. For purposes of simplicity, these will collectively be referred to as IM products. These IM products can cause undesirable interference over the amplifier frequency range. It is for these reasons that most amplifiers must incorporate some form of correction mechanism into the signal flow path through the amplifier.
One conventional method for reducing IM products uses a spectral analysis approach. This approach involves scanning the output of the amplifier using a receiver tuned to the frequency of candidate IM products. The IM level is measured and a linearizer is manually adjusted to minimize the IM product. This procedure is repeated until each of the IM products have a magnitude below a predetermined acceptable level. In a similar approach, as described in U.S. Pat. No. 4,580,105 issued to Myer, a portion of the output signal is taken and combined with an input signal which has been adjusted in phase and amplitude. The signal combination isolates the distortion component which is then adjusted in phase and gain. This isolation distortion component is then added back to the output signal by means of a coupler to eliminate the distortion component. Unfortunately, the amount of distortion reduction available using this feed forward technique is limited by the accuracy of the gain and phase adjustments.
In addition, there appears to be a problem in the number of samples realized in several basic feed forward techniques. Samples in these systems are available only at the time that the phase detector circuit detects a correlated signal. This leads to poor error signal resolution and it is this error signal that is used to cancel corresponding spurs.
Moreover, it is believed that many cellular communication base stations do not function at full capacity because of serious linearity problems in the base station's components. In addition, the need for increased capacity accompanying the expansion of cellular communications is forcing requirements for transmission standards of RF equipment to become too stringent to be met by construction of most present day amplifier circuits. Moreover, both time division multiple access (TDMA) and code division multiple access (CDMA) modulation require greater linearity and an absence of IM products that can not be routinely obtained by presently available high efficiency RF power amplifiers. Accordingly, it is clear that present correction techniques for eliminating IM products are not acceptable for evolving communications systems.
Thus, there is a need for a cancellation circuit which reduces IM products in electronic circuitry such as power amplifiers.
It would also be highly advantageous if the IM cancellation circuit were inexpensive to manufacture and highly reliable.
Moreover, it would be highly advantageous if the IM cancellation could be provided in a compact and lightweight constriction.